Coal washing plants are provided near mineheads in order to separate combustible coal from non-combustible and usually denser rock and shale.
A variety of apparatuses have been proposed for this purpose, including inclined rotating sieve screens, centrifugal washers, and inclined rotating barrels, all relying on some form of centrifugal classification of material in accordance with specific gravity.
The present invention relates to the inclined barrel type of coal washing plant. Essentially, the known plant comprises an elongate rotating barrel, the axis of which is inclined at about 15.degree. to the horizontal. Raw coal is fed into the upper end of the barrel, and the coal is washed with wash water from a launder pipe. The material is classified so that shale and other dross leaves the barrel from the upper end, and concentrated coal at the lower end. The coal is sorted by a screen, through which fines fall, and water drains off, and the fines and water are recirculated into the top of the barrel.
The through put rate of such a plant is in the order of 20 tons per hour.
The invention: It is an object to increase the through put rate, so that a much larger quantity of coal can be washed, with a consequent increase in efficiency.
The coal washing plant comprises an inclined rotatable barrel, with means for feeding raw coal to its upper end, an outlet associated with the upper end for disposal of shale, a screen at the lower end for screening washed coal, means for removing the washed coal, the screen having a first finer region over a first wash liquor recovery tank, and a second coarser region over a second wash liquor recovery tank, the second tank having means for recycling wash liquor and fines to the barrel, and the first tank having means for feeding wash liquor and fines to a secondary separation stage preferably comprised by one or more cyclone separations.
The provision of a secondary, separation stage enables the through put of the barrel to be considerably increased, as the decreased efficiency of separation in a barrel working at a higher rate is compensated by the greater efficiency of the cyclone separator in separating coal fines when treating small volumes of borderline material from shale fines.
Two cyclone separators may be provided to be operated together or alternatingly in parallel, The separators discharge shale from one end, and coal from the other, and preferably the or each separator includes a vortex finder to enable an exact separation to be achieved.